WASHINGTON (CNN) -- Efforts to pass a constitutional amendment that would effectively ban same-sex marriage failed in the Senate Wednesday afternoon, but supporters vowed to keep fighting for the measure.

"This is a long process," said Republican Sen. Wayne Allard of Colorado, sponsor of the amendment. "Nobody on our side, I think, ever felt for a minute that this was going to be a one-shot deal and it was going to be over with at that particular point in time."

The White House released a statement from President Bush in which he said he was "disappointed" that the amendment was "temporarily blocked" in the Senate and urged the House to take up the matter.

"Activist judges and local officials in some parts of the country are not letting up in their efforts to redefine marriage for the rest of America, and neither should defenders of traditional marriage flag in their efforts," Bush said in his statement.

The proposed amendment, championed by Bush, was killed for this session after a procedural vote to move the measure to the Senate floor for final consideration failed 48-50 -- 12 votes shy of the 60 required by Senate rules.

Six Republicans -- including Sen. John McCain of Arizona -- joined 43 Democrats and one independent to defeat the measure. Three Democrats and 45 Republicans voted for it.

Republicans had expected to muster the votes needed to at least advance the measure, if not the 67 required to pass it. They also expected to force the presumptive Democratic presidential ticket -- Sen. John Kerry of Massachusetts and Sen. John Edwards of North Carolina -- to vote against it.

A constitutional amendment requires a two-thirds majority of both houses of Congress to pass. Then the proposal would need the approval of three-fourths of the state legislatures to be ratified.

Opponents denounced the failed effort as a "political tool" during an election year.

"Today, we saw President Bush and the Republican leadership attempt to divide America and it backfired, instead dividing their own party," said Cheryl Jacques, president of the Human Rights Campaign, a gay rights organization. "We saw the politics of distraction fail and fail handily."

Both Kerry and Edwards were on record opposing the measure but decided not to return for the procedural vote since their votes weren't needed to defeat it. They were the only senators not voting.

Kerry, who was in Boston, issued a statement saying the Senate floor "should only be used for the common good, not issues designed to divide us for political purposes."

Edwards, at a campaign rally in Iowa, said "the president and the vice president tried to use our Constitution and the amendment of that Constitution as a political tool, and the United States Senate, they said, 'No. We will not accept it.' "

A Bush campaign aide responded, "It takes a special kind of senator to attack others over a vote that they don't show up for."

Bush did not directly address the amendment's defeat during a bus tour of Wisconsin, but he reiterated his opposition to same-sex marriage during a rally in Ashwaubenon, a Green Bay suburb.

"We stand for institutions like marriage and family which are the foundations of our society," he said, drawing thunderous applause from the partisan crowd. "We stand for judges who strictly and faithfully interpret the law, instead of legislating from the bench."

Social conservatives have been pushing hard for the measure since May, when the highest court in Massachusetts legalized same-sex marriages in the Bay State.

Polls show a solid majority of Americans are against legalizing same-sex marriages, although the gap narrows when it comes to amending the Constitution.

Varying views

Bush's stance against was echoed by Republican Sen. Bill Frist of Tennessee during debate. "Will activist judges not elected by the American people destroy the institution of marriage, or will the people protect marriage as the best way to raise children? My vote is with the people," said the majority leader.

Republican Sen. Orrin Hatch of Utah said the amendment would simply preserve a fundamental institution "that a few unelected judges are trying to radically change." It's not a question of discrimination against gays, he said.

The amendment, as proposed by Allard, would add these two sentences to the Constitution:

"Marriage in the United States shall consist only of the union of a man and a woman. Neither this Constitution, nor the constitution of any State, shall be construed to require that marriage or the legal incidents thereof be conferred upon any union other than the union of a man and a woman."

Some Republicans objected to the second sentence, saying it was so ambiguous that it also could prevent states from allowing gays and lesbians to join in civil unions.

Other senators expressed concern that the measure would usurp the states' traditional dominion over family law, and some questioned whether it was necessary.

Republican Sen. John Sununu of New Hampshire, who voted against moving the measure forward, said it was too early to make the assumption that judges might strike down laws such as the 1996 federal Defense of Marriage Act and 38 similar state statutes that define marriage as a union only between a man and a woman.

"Naturally, there exist concerns about what activist courts might do to undermine these rights and the Defense of Marriage Act," Sununu said in a statement. "But it is premature to amend the Constitution based upon a hypothetical scenario."

McCain went even further, calling the amendment "antithetical in every way to the core philosophy of Republicans." (Full story)

Besides Sununu and McCain, the other Republicans who broke with the GOP leadership on the issue were Olympia Snowe and Susan Collins of Maine, Lincoln Chafee of Rhode Island and Ben Nighthorse Campbell of Colorado.

The three Democrats voting to advance the measure were Robert Byrd of West Virginia, Zell Miller of Georgia and Ben Nelson of Nebraska.

Afterward, proponents tried to put the best face on the defeat, vowing to press forward until they win.

"I think we are going to have a long and extended discussion in the country about what is marriage. ... We won on substance. We lost on procedure," said Republican Sam Brownback of Kansas, noting that Democrats were "definitely" not listening to their constituents.